LM61460EVM: Not able to adjust 20V to 13.5V in the EVM

Brian Han1

Part Number: LM61460EVM
Other Parts Discussed in Thread: LM61460

Hello,

I'm trying to make a 13.5V with LM61460, but the actual test using the EVM is giving me different results.

I only changed Rt and Rfbb values of the EVM and nothing else.

I started out with the default EVM settings and the Vout was correct. but When I adjusted the Rfbb value to make 13.5V Vout, the EVM is giving me a Vout of near Vin.

Vin	Vout Target	Rt	Rfbb	Actual Vout	Remark
20V	5V	33.2k	24.9k	5V	Default Setting, 400kHz switching
20V	13.5V	33.2k	8k	19.8V	Adjusted Rfbb value from the WEBBENCH 13.5V output setting
20V	13.5V	10k	8k	19.8V	changed switching to 1.29MHz
20V	-	10k	10k	11.2V	Min Rfbb value that actually changes Vout

Is 13.5V Vout not possible from 20V Vin?

Best regards

Brian

over 3 years ago

+1 Samuel Jaffe over 3 years ago

TI__Mastermind 25590 points

Brian,

13.5VOUT is definitely possible. This device regulates the output to set the voltage on FB to 1V. Check that FB is at 1V. If it is, the device is operating properly and you should double check your resistors.

If the switch node is unstable or skipping pulses, you may need to adjust the power stage.

-Sam

0 Brian Han1 over 3 years ago in reply to Samuel Jaffe

Intellectual 530 points

Hi Sam,

Thanks for the suggestion. I re soldered the Rfbb resistor and changed the power supply from SMPS to a linear one and now it's working.

Best Regards

0 Brian Han1 over 3 years ago in reply to Samuel Jaffe

Intellectual 530 points

Hi Sam,

I made my custom board and I'm trying to do the same as the EVM(20V -> 13.5V), but I'm getting near 0V output from the LM61460.

There are some differences between the EVM and my custom board, but I don't think it's big enough to cause this issue.

I didn't need PGOOD, so I left it out. I connected BIAS pin directly to the ground because my output was 13.5V, which is over 12V

Is there anything that I could check whether the schematic is right or wrong?

Measurements.

1. RT - Device frequency was set to be around 13MHz soI'm using 10k.

2. FB - As you have mentioned before, the FB needs to be around 1V, but this point is very low near 0 and there's an unknown peak noise every 100ms

3. RBOOT - When I measure RBOOT, CBOOT, the voltage reading is around 3.3V, but there's24V sharp peaks very frequently, but I'm not sure if this is a normal behavior.

If you see anything wrong with my schematics or the measures, Please let me know.

Best Regards

Brian

+1 Samuel Jaffe over 3 years ago in reply to Brian Han1

TI__Mastermind 25590 points

Brian,

It looks like you are in hiccup mode from a short circuit on the output. Hiccup mode senses if VOUT falls below 40% of the set regulation. If that condition is true for 128 switch cycles, the device stops switching for ~80ms then tries again. Check the output for short circuits.

Also a note: This is set to 1.3MHz, not 13MHz.

-Sam

0 Brian Han1 over 3 years ago in reply to Samuel Jaffe

Intellectual 530 points

Hi Sam,

You were absolutely correct. It was from the short circuit, which I didn't expect at all.

The newly SMT boards didn't have any short circuit before the power was supplied, But as Soon as I supplied the board with power, it shutdown and restarted.

My board is actually using 2 LM61460s and it converts 20V to 13.5V in the first stage and converts 13.5V to -13.5V in the second stage. And I found out that the Second LM61460 IC, which converts 13.5V to -13.5V, was damaged and causing short.

Is there anything wrong with my design? I actually asked in E2E forum about the design and simulation and modified my design accordingly. LM61460-Q1: Vin decreases in TINA simulation with inverted Vout

Best Regards,

Brian

0 Brian Han1 over 3 years ago in reply to Samuel Jaffe

Intellectual 530 points

Hi Sam,

Would it be possible to check my buck boost schematic and see if there's any problem with it?

Thanks

0 Samuel Jaffe over 3 years ago in reply to Brian Han1

TI__Mastermind 25590 points

Brian,

On quick inspection it looks like you're missing a top feedback resistor. C58 and R26 should be in parallel with a resistor to set the output voltage.

-Sam

0 Brian Han1 over 3 years ago in reply to Samuel Jaffe

Intellectual 530 points

Hi Sam,

So do you mean the feedback resistor should have been like this?

+1 Samuel Jaffe over 3 years ago in reply to Brian Han1

TI__Mastermind 25590 points

Brian,

Yes that is the correct placement but you may also want a resistor in series with the 18pF capacitor. See the EVM schematic.

-Sam

0 Brian Han1 over 3 years ago in reply to Samuel Jaffe

Intellectual 530 points

Thank you Sam, everything is running perfectly!

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LM61460EVM: Not able to adjust 20V to 13.5V in the EVM